With an injection molding machine, products are mass-produced by repeating an injection molding cycle in which molten molding material is injected in a mold, and is cooled under a hold pressure, and is further cooled and solidified. Conventionally, to judge whether mass-produced products are acceptable or not, a product sampled from each product lot is visually compared with a nondefective sample manufactured prior to mass production, thus judging whether the products of each lot are acceptable or not. This judgment method, however, is disadvantageous in that the judgment reliability is low because the acceptance/rejection judgment is made by visual inspection and also because such nondefective samples may be subjected to change on standing. Further, a large space is required to keep many nondefective samples.
The present inventors perceived that the quality of products depends on the changing pattern of the pressure applied to molding material during the injection and hold processes, and proposed and article conformity discriminating apparatus in Japanese Patent Application S63-118357 (International Patent Application PCT/JP90/00396). According to the proposed apparatus, the changing pattern of the reference pressure during a period from the start of injection to the completion of hold is predetermined based on the pressures (reference pressures) which are sampled at regular intervals during the injection and hold processes in nondefective product molding. And an actual pressure change pattern is determined in accordance with the pressures sampled periodically during actual injection and hold processes. Then, it is judged whether differences between the pressures at individual matching points of the two patterns are out of permissible ranges or not, and then it is decided whether the product is acceptable or not according to the number of occurrences of actual pressures that are out of the permissible ranges and are therefore inappropriate.
The present inventors also proposed a product acceptance/rejection judgment system in Japanese Patent Application H1-130064. In the proposed system, for instance, an actual injection pressure is detected at a preset time period elapsed from the start of injection, or actual injection pressures are detected when a screw reaches two preset screw positions after the injection start, respectively. Then, the deviation of each actual pressure from the predetermined pressure is checked whether it falls within a permissible range or not, thus determining whether the product is acceptable or not.
As described above, in these proposals, the actual pressure is detected once or plural times at each detection timing which is determined by an injection control/judgment standard parameter consisting of the elapsed time period from the start of injection or the screw position. The deviation of each actual pressure from its corresponding reference pressure or preset pressure is judged whether it falls within a permissible range or not, then a determination is made as to whether the product is acceptable or not in accordance with the judgment result.
There are some cases, however, that the above two proposals fail to perform accurate acceptance/rejection judgment of products. This is considered primarily because a permissible pressure range (judgment standard parameter) is fixedly set irrespectively of injection control and judgment standard parameter values, despite the fact that the degree of affection of a pressure deviation on the conformity of a product varies with changes in the aforementioned parameter values, i.e., with the progress of injection/hold process. Namely, in the proposed apparatus according to Japanese Patent Application S63-118357, a determination is made as to whether the pressure deviation at each point of a pressure change pattern falls within a fixedly set permissible range. In other words, no consideration is given to the difference between affection degrees of the pressure deviations at various points in the pattern. Nevertheless, the judgment results at the various points in the pressure change pattern are evaluated with the same weight, leading to a possible inappropriate acceptance/rejection judgment of a product. Also in the proposed system of Patent Application H1-130064, which determines whether the deviation between each of the actual pressures detected twice and the corresponding preset pressure falls within the permissible range, normally, all the permissible ranges are set in the same manner. In this proposed system, it appers possible to independently set the respective permissible pressure deviation range in dependence on a parameter value (actual pressure detection timing), so as to eliminate the abovementioned drawback resulting from evaluating all the judgment results with the same weight. However, in practice, an extreme difficulty will be encountered in deciding a preset pressure and a permissible pressure deviation range that are suited for the parameter value, with reference to past molding work data. Similarly, it is also difficult to set the permissible range in dependence on the parameter value in the aforementioned proposed apparatus.